Polyesters are widely used in a variety of industries. For polyester synthesis, a titanium-based catalyst is commonly used, which shows a superior catalytic activity over other metal and non-metal catalysts.
Among a variety of polyester applications, some commercial products composed of polyester are made or used in an aqueous solution. An example of aqueous application is a polyether-polyester block copolymer which has been used in fabric mill treatment and in both powder and liquid detergent as soil release polymer (SRP). For example, polyethylene terephthalate/polyoxyethylene terephthalate (PET/POET) and its variants are used as soil release agents in liquid detergent compositions. These copolymers are described in U.S. Pat. Nos. 3,416,952, 4,349,688, 4,702,857, 4,877,896, 4,738,787, and 5,786,318 etc. When a polyester product is dissolved in water, trace of titanium residue left from the reaction will precipitate out of the solution, especially after several months when firstly dissolved. This will deteriorate the product quality and thus is not desired for a commercial product, especially for those long-term storage liquid reagents.
Although there are many literatures for titanium-catalyzed polyester synthesis, there are a very limited number of literatures for removing the residual titanium, especially for removing the residual titanium from aqueous product.
Chinese patent CN1283614C discloses a process for removing titanate catalyst from an esterification product, in which titanate catalyst was treated with sodium percarbonate, a mixture of sodium carbonate and an aqueous solution of hydrogen peroxide, or a mixture of sodium bicarbonate and an aqueous solution of hydrogen peroxide to form a flocculent precipitate, followed by filtering the resultant precipitate to remove the titanate catalyst. During the reaction, an alkali condition was utilized.
PCT International Publication No. WO 2001019775 discloses a process for removing titanium oxide from a polyester decomposition product, in which the titanium oxide was reacted with calcium oxide, calcium carbonate or calcium hydroxide to form titanium dioxide, and the formed titanium dioxide was aggregate and filtered. During the reaction, an alkali condition was also applied.
However, the methods described in above patent/patent application fail to teach that the titanium residue can be reduced to a satisfactory level, for example, of less than 5 ppm. In addition, there is a risk of hydrolysis for polyester when exposing the polyester product in a strict alkaline reaction condition.
Furthermore, filtering the titanium residue from aqueous solution is labor-consuming and time-consuming. All above references fail to teach the inorganic form of titanium can be effectively removed by filtration within an acceptable period of time, for example, within 4 hours.
As a result, a safe and effective method of reducing level of titanium in an aqueous solution of polyester is needed to be explored.